169
FOUS30 KWBC 270023
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
823 PM EDT Sun May 26 2024

Day 1
Valid 01Z Mon May 27 2024 - 12Z Mon May 27 2024

...A MODERATE RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
KENTUCKY, NORTHERN TENNESSEE, SOUTHERN ILLINOIS/INDIANA, AND
SOUTHEAST MISSOURI...

The Moderate risk still looks in pretty good shape across portions
of the OH Valley where numerous instances of flash flooding are
expected into tonight. Convection continues to grow upscale over
portions of southeast MO and southern IL this evening, and is
expected to develop into a squall line that will push southeast
across KY and TN tonight. The squall line will likely become
progressive in nature, however intense rainfall rates will still
allow for a quick 1-2" of rainfall. Recent rainfall has saturated
soil conditions and lowered FFG across the the MDT risk
area...thus this additional rainfall is likely to result in some
flash flood impacts. Given the strong low level jet and increasing
forcing along/ahead of the developing squall line, expect we will
continue to see some discrete supercells develop ahead of the
convective line. Where these cells merge into the squall line there
will be localized areas where the duration of heavy rainfall is
extended. If this occurs over any more sensitive basin or urban
area then areas of significant flash flooding could evolve. The
MDT risk was expanded a bit north to account for
model/observational trends.

Chenard


Day 2
Valid 12Z Mon May 27 2024 - 12Z Tue May 28 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE NORTHERN MID-ATLANTIC STATES...

...2030Z Update...
A deep cyclone for the month of May will continue to lift into
southeast Canada. Circulation around the low will continue to draw
moisture northward over portions of the eastern U.S.. The 12Z run
of the models were not showing quite as much instability and
somewhat lower precipitable water values...but still mesoscale
forcing to keep the Slight Risk with just a couple minor
adjustments. Elsewhere...the placement of the Marginal still
adequately covers the uncertainty of where heavy rainfall may
occur given the instability and heard-to-time shortwave energy.

Bann


...0830Z Excessive Rainfall Discussion...

...Great Lakes/East...
A cyclone on the deep/strong side of May climatology lifts into
southeast Canada. A broad area of 1.5"+ precipitable water values
advects in ahead of the system and focuses along the system`s
frontal boundaries and within area terrain. The Slight Risk for
portions of PA and NY is similar to continuity. There is a 6-12
hour period where Atlantic moisture advects in while the 850 hPa
flow remains quasi-stationary and fairly strong, which could lead
to a decent period of moderate to heavy rainfall. In this area,
hourly rain totals to 2.5" and local amounts to 5" are considered
possible where cells merge, train, or where mesocyclones form.

As 700 hPa temperatures are modest, heavy rainfall potential is
there along its cold front as it sweeps through the Cumberland
Plateau, the Appalachians, and the Southeast. There is some concern
that MCVs from previous activity in the Midwest and Mid-South
could sail into the area to increase divergence aloft, effective
bulk shear, and thunderstorm organization across the Southeast, so
broadened the Marginal Risk in this area. There`s even some
convective potential within the system`s comma ahead across
portions of MI; the guidance shows a narrower range of possible
outcomes compared to this time yesterday, which appears to be due
to timing differences. Within the Marginal Risk area, hourly rain
totals to 2" and local amounts to 4" are possible where cells
merge, train, or mesocyclones form.

Roth


Day 3
Valid 12Z Tue May 28 2024 - 12Z Wed May 29 2024

...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
TEXAS AND SOUTHERN OKLAHOMA...

...2030Z Update...
The 12Z suite of guidance continues to favor portions of Texas and
southern Oklahoma for another round of convection capable of
producing heavy to potentially excessive rainfall. Return flow
draws moisture back into the region...setting the stage for
convection to develop late Tuesday afternoon and Tuesday evening as
a mid-level shortwave trough crosses Texas from west to east.
Model agreement was still fairly good allowing the Slight Risk to
stay in place without too many adjustments.

Bann


...0830Z Excessive Rainfall Discussion...

Weak shortwaves rippling through and dampening mid-level ridging
across TX are expected to force a return flow of moisture and
instability back into the western Gulf Coast. Precipitable water
values of 1.5-1.75" are forecast, and low-level inflow and
effective bulk shear should be just sufficient for convective
organization near a front. The 00z NAM advertises MU CAPE of 5000+
J/kg, so there`s instability to spare. While 700 hPa temperatures
are plenty warm, implying a decent mid-level cap, the front should
be an effective focus and allow the cap to break. Most of the
guidance shows reasonable agreement, with the exception of the 00z
GFS that streaks heavy rainfall eastward. Since it can be prone to
convective feedback which can lead to such an evolution, decided to
account for its solution in the Marginal Risk area by showing some
eastward expansion. The inherited Slight Risk resembles continuity.

Roth


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt